Anechoic chamber absorber and method

ABSTRACT

An EMR-absorbing anechoic chamber absorber (22, 42) uses a block of honeycomb material with a lossy material, such as carbon, applied to the sheets (4) of material from which the honeycomb is made to create wedges (36) or pyramids (44) of lossy material. Preferably, the lossy material is applied to the sheets of material in a repeating wedge-shaped pattern (16) to create the wedges of lossy material within the solid rectangular block of honeycomb material. Parallel, wedge-shaped grooves (43) are formed in the block of honeycomb material to create the pyramids of lossy material. If the lossy material is applied to the entire sheet, a set of parallel, wedge-shaped grooves are formed in the block of honeycomb material to create wedges of lossy material; two sets of parallel, wedge-shaped grooves are formed at 90° angles to one another to create pyramids of lossy material.

This application is a division of application Ser. No. 08/368,785, filedJan. 4, 1995, now U.S. Pat. No. 5,594,218.

BACKGROUND OF THE INVENTION

Anechoic chambers typically have their interior surfaces covered withwedge-shaped and pyramid-shaped blocks of polyurethane foam injectedwith carbon. Carbon acts as a lossy material absorbing electromagneticradiation (EMR). These foam blocks are typically 6 feet (1.8 m) tallwhen testing in certain frequency ranges, such as 200 MHz to 500 MHz andup. The wedge-shaped blocks are used along the side walls, ceiling andfloor while pyramid-shaped blocks are used at either end of the room,that is, behind the radiation source and behind the target.Pyramid-shaped blocks are used whenever radiation is expected to strikethe surface generally straight on. Therefore the pyramid-shaped blocksare also used along the side walls near each end of the chamber.

Conventional foam absorbers, in particular the pyramid-shaped absorbers,are quite heavy, can sag (reducing their effectiveness and requiringreplacement) and take up a substantial amount of space. The large sizeof the foam wedges and pyramids forces the use of a great deal ofabsorber material and a large room to create only a modest size chamber.The weight of the foam absorbers mandates that the walls and ceiling bemade quite strong, thus increasing the cost of construction.

SUMMARY OF THE INVENTION

The present invention is directed to an anechoic chamber absorber, andmethod for making the anechoic chamber absorber from honeycomb, which isrelatively lightweight, stiff, has low toxicity in the event of fire andcan be made to be about one-half the height of conventional foam blockabsorbers. Reducing the size of the absorber can drastically reduce thedimension requirements for the basic anechoic chamber room Structure andreduces the amount of absorber material which must be used by about 50%.

The anechoic chamber absorber suitable for absorbing electromagneticradiation is made using a block of honeycomb material. The honeycombmaterial includes wedges or pyramids of a lossy material, such ascarbon. The wedges and pyramids can be formed within a rectangular blockof honeycomb without the need for machining.

The lossy material is preferably applied to sheets of material fromwhich the honeycomb is made. If the lossy material is applied to theentire sheet, the wedges or pyramids of lossy material are created byforming a set of parallel, wedge-shaped grooves in the block ofhoneycomb material to create wedges of lossy material. Two sets ofparallel, wedge-shaped grooves formed at 90° angles to one another areused to create pyramids of lossy material.

The lossy material is preferably applied to the sheets of material in arepeating wedge-shaped pattern. On forming the block of honeycombmaterial, wedges of lossy material are formed within the solidrectangular block of honeycomb material without the need for cuttingwedge-shaped grooves into the block. To create pyramids of lossymaterial, wedge-shaped grooves of the honeycomb material are removedfrom the block of honeycomb material. The grooves are cut at 90° anglesto the wedges of lossy material so that truncated wedge-shaped sectionsof lossy material are removed to create a grooved block of honeycombmaterial having pyramids of lossy material within the grooved block.

A primary advantage of the invention is the ability to reduce the sizeof anechoic chamber absorbers from, for example, 6 feet (1.8 m) to 3feet (0.9 m) tall when dealing with EMR in the range of 300 MHz to above1 GHz. This provides a substantial cost saving both as to the size andconstruction of the room and the amount of material needed. Reducing theweight of the absorber material reduces construction costs since thewalls of the room need not be made extra strong to support the weight ofconventional foam absorbers. Using honeycomb absorber materials alsoallows one to customize the absorber properties when desired.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a stack of sheets of material with a wedge-shapedpattern of lossy material applied to each sheet;

FIG. 2 shows a block of honeycomb material, made from a stack of thesheets of material of FIG. 1, having wedges of lossy material;

FIG. 3 illustrates the block of honeycomb material of FIG. 2 with wedgesof honeycomb material removed from the block to create a grooved blockof honeycomb material having pyramids of lossy material; and

FIG. 4 shows a stack of sheets of material with wedge-shaped patterns oflossy material configured to create pyramids of lossy material withoutany cutting or grooving of the resulting block of honeycomb material asis required with the embodiment of FIGS. 1-3;

FIG. 5 illustrates a plot of conductivity versus height for X- andZ-dependent conductivity;

FIG. 6 illustrates a plot of conductivity versus height for Z-dependentconductivity;

FIG. 7 plots reflection versus frequency for both a conventional 72"tall foam pyramid absorber and a 36" tall pyramid absorber madeaccording to the present invention; and

FIG. 8 plots calculated conductivity versus distance from tip.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a stack 2 of sheets of material 4, each sheet havinga repeating wedge-shaped pattern of lossy material 6. Each sheet 4 has alength 8 and a height 10, height 10 extending between first and secondedges 12, 14 of sheet 4. Each pattern 6 of lossy material is made up ofwedge-shaped units 16 of the lossy material. Each wedge-shaped unit 16has a narrow tip portion 18 adjacent first edge 12 and a broadened baseportion 20 adjacent second edge 14. In the preferred embodiment,material 4 is preferably Kraft paper or Nomex™ while the lossy materialis preferably carbon. Other lossy material, such as nichrome or siliconcarbide, could also be used.

Stack 2 of sheets of material 4 are then formed into a block ofhoneycomb material 22 shown in FIG. 2. Honeycomb material is usuallymade in one of two ways. One way is to take flat sheets of material andcorrugate it into half honeycomb shapes and then glue these corrugatedsheets to one another. The other way is to take sheets of material andput glue lines on them. The glue lines between one set of sheets areoffset from the glue lines between the sheets on either side. When thesheets are adhered to one another and the stack of sheets is expanded,an expanded honeycomb type of material results. The method of making theblock of honeycomb material 22 is not part of this invention. Variousmethods for making honeycomb material are discussed in U.S. Pat. No.5,312,511 to Fell, the disclosure of which is incorporated by reference.

Block 22 includes L and T directions 24, 26 corresponding to length 8and height 10 of material 4. Block 22 also includes a W direction 28which corresponds to the thickness of the stack 2 of material 4. Asshown in FIG. 2, block 22 of honeycomb material is essentially a solidblock with the hexagonal openings 30 extending from the top side 32 tothe bottom side 34. The aligned stacks of wedge-shaped units 16 of lossymaterial form wedges 36 of lossy material as shown in FIG. 2. The wedges36 of lossy material extend between the front side 38 and the back side40 of block 22. Block 22 of honeycomb material, containing wedges 36 oflossy material, are useful in covering the sidewalls, ceiling and floorof anechoic chambers.

In this preferred embodiment, hexagonal opening 30 is about 0.28" wide.Block 22 is designed to absorb EMR in the 300 MHZ to above 1 GHzfrequency range. Block 22 has a height in H dimension 26 of about 3 feetwith the spacing between wedges 36 of about 14 inches. The height ofblock 22 is about half the height that a conventional carbon-loaded foamanechoic chamber absorber would be for the same frequency range. Theweight of block 22 is about 10% of the weight of a conventionalcarbon-loaded polyurethane foam anechoic chamber absorber. Therefore,the anechoic chamber itself can be smaller because of the lesser heightof block 22 and the walls and ceiling of the anechoic chamber need notbe made as strong as with conventional absorbers because of the lowerweight.

The use of carbon-loaded honeycomb as an absorber is described in U.S.patent application No. 07/890,757, titled Method for Making Materialwith Artificial Dielectric Constant, now U.S. Pat. No. 5,385,623, thedisclosure of which is incorporated by reference. Carbon, or other lossymaterial, can be applied in a pattern or over the entire lossy surfaceaccording to the absorber characteristics desired.

FIG. 3 illustrates the result of cutting wedge-shaped sections ofmaterial from block 22 to create a grooved block 42 of honeycombmaterial. Groove block 42 has wedge-shaped open regions 43 extendingfrom top side 32 of block 22 down towards but not all the way to bottomside 34. Doing so results in creating pyramids 44 of lossy material.Groove block 42 can thus be used where pyramids of lossy material areneeded, such as behind the radiation source and behind the target. Theymay also be used on the ceiling, sidewalls and floor adjacent to thefront wall and back wall. Block 42 could be further modified to removethose portions of the material which do not contain lossy material.However, unless weight is a key concern, it is not expected to benecessary to do so.

FIG. 4 illustrates a stack 46 of sheets of material 48 similar to thoseshown in FIG. 1 but in which successive sheets have increasingly largerwedge-shaped units 16a, 16b . . . of lossy material formed on sheets 48.Doing so allows the user to create pyramids of lossy material withoutthe need for any sort of cutting as is required to create pyramids oflossy material from the wedges of lossy material in block 22 of FIG. 2.

Material Design Process For The Preferred Embodiment

Designing the material involves a systematic adjustment of conductivityprofiles and gradient parameters within a constrained design space.

FIG. 7 illustrates the design space and associated variables availableto optimize the absorber performance. Both the pyramidal region and thebase have z-dependent conductivity profiles. The pyramidal region isallowed two-dimensional (x,z) conductivity gradients plus modulation ofthe basic pyramidal shape (h(x,z)).

    σ=σ.sub.0  r.sub.0 +r.sub.2 Z.sup.(P.sbsp.1.sup.+c sin (3πZ))P.sbsp.2 !

where:

r₀ =tip conductivity factor

r₂ =conductivity scaling factor

p₁ =profile compensation factor

c=sinusoidal ripple coefficient

p₂ =Z dependent power profile

σ₀ =initial conductivity in Siemens/m.

FIG. 7 illustrates an initial conductivity design space offering maximumdegrees of freedom. Both x-dependent and z-dependent conductivitygradients are possible.

For the preferred embodiment of straight sided pyramids, the equationreduces to

    σ=σ.sub.0  r.sub.0 +r.sub.z Z.sup.p2 !

where:

r₀ =tip conductivity factor

r₂ =conductivity scaling factor

p₂ =Z dependent power profile

σ₀ =initial conductivity.

FIG. 6 shows a reduced conductivity design space having onlyz-dependence in the pyramidal region. In this reduced design space, theperformance of the absorber is influenced by the initial tipconductivity, final pyramidal conductivity, the profile by which theconductivity increases from its initial to final value, and theconductivity of the base region. FIG. 7 shows the conductivity profilefor the preferred embodiment designed to operate from 300 MHz upwards.FIG. 8 shows the calculated performance.

Modification and variation can be made to the disclosed embodimentswithout departing from the subject of the invention as defined in thefollowing claims. For example, in the preferred embodiment, wedges 36and pyramids 44 of lossy material are shown having flat sides andextending to relatively sharp tips; they could have convex, concave orirregularly-shaped sides and need not terminate in a sharpened tipaccording to the performance desired, manufacturing requirements and userestrictions. Instead of honeycomb having hexagonal openings, the blocksof honeycomb material could be made from other types of open cellularmaterial having other shapes of openings. The honeycomb material couldbe made with, for example, composite skins bonded to top and bottomsides 32, 34 thus sealing openings 30; this adds structural strength,facilitates stacking of the absorbers and keeps the interiors of thehoneycomb cells sealed to prevent collection of charged dust particleswhich could change the electrical properties of the absorber.

What is claimed is:
 1. A method of making an anechoic chamber absorbercomprising the following steps:selecting sheets of material each havingfirst and second edges defining a length and a height; applying a lossymaterial to the sheets of material in a repeating wedge-shaped pattern,said wedge-shaped pattern including periodic wedge-shaped unitsextending along the length of the sheets of material, the wedge-shapedunits having narrow tip portions facing the first edge and broadenedbase portions facing the second edge; and creating a block of honeycombmaterial from the sheets of material with said lossy material applied inthe wedge-shaped pattern thereby creating wedges of lossy materialwithin the block of honeycomb material, said block of honeycomb materialhaving an T-direction and a L-direction, corresponding to the height andlength of the sheets of material, and a W-direction.
 2. The methodaccording to claim 1 wherein the applying step is carried out with thenarrow tip portions extending to the first edges.
 3. The methodaccording to claim 1 wherein the applying step is carried out byapplying carbon as the lossy material.
 4. The method according to claim1 wherein the applying step includes the step of selecting thewedge-shaped units with straight sides between the tip portion and thebase portion.
 5. The method according to claim 1 further comprising thestep of removing wedges of honeycomb material from the block ofhoneycomb material in the L-direction to create a grooved block ofhoneycomb material thereby creating wedges of honeycomb materialextending in the L-direction and pyramids of lossy material within thewedges of honeycomb material.
 6. The method according to claim 5 whereinsaid removing step is carried out so that said wedges of honeycombmaterial have pointed tips and flat sides.
 7. A method of making ananechoic chamber absorber comprising the following steps:selectingsheets of material each including lossy material and having first andsecond edges defining a length and a height; creating a block ofhoneycomb material from the sheets of material; and creating wedges oflossy material within the block of honeycomb material, said block ofhoneycomb material having an T-direction and a L-direction,corresponding to the height and length of the sheets of material, and aW-direction.
 8. The method according to claim 7 wherein the creatingstep includes the step of applying a lossy material to sheets ofmaterial in a repeating wedge-shaped pattern, said wedge-shaped patternincluding periodic wedge-shaped units extending along the length of thesheets of material, the wedge-shaped units having narrow tip portionsfacing the first edge and broadened base portions facing the secondedge.
 9. The method according to claim 8 further comprising the step ofremoving wedge-shaped sections of honeycomb material from the block ofhoneycomb material in the L-direction to create a grooved block ofhoneycomb material thereby creating wedges of honeycomb materialextending in the L-direction and pyramids of lossy material within thewedges of honeycomb material.
 10. The method according to claim 7further comprising the step of removing wedge-shaped sections ofhoneycomb material from the block of honeycomb material in theL-direction to create a grooved block of honeycomb material.
 11. Themethod according to claim 10 wherein the removing step creates wedges ofhoneycomb material extending in the L-direction and pyramids of lossymaterial within the wedges of honeycomb material.
 12. A method of makingan anechoic chamber absorber comprising the following steps:selectingsheets of material each having first and second edges defining a lengthand a height; applying a lossy material to the sheets of material in arepeating wedge-shaped pattern, said wedge-shaped pattern includingperiodic wedge-shaped units extending along the length of the sheets ofmaterial, the wedge-shaped units having narrow tip portions facing thefirst edge and broadened base portions facing the second edge;periodically increasing and decreasing the heights of the wedge-shapedunits applied to successive ones of the sheets of material; and creatinga block of honeycomb material from the sheets of material with saidlossy material applied in the wedge-shaped pattern thereby creatingpyramids of lossy material within the block of honeycomb material.